Blockposters.com was sent to me by a fellow teacher and it looks like a winner. It's a very simple 3 step process.
1. Upload the image of your choice.
2. Decide how big you want the poster to be.
3. Download your poster as a PDF.
The website chops up the picture into seperate pages that you can print our and assemble yourself. Student projects or class rules could be turned into larger posters you can hang in your room. Have a look around and let me know what you think.